eCryptfs: Privileged kthread for lower file opens

eCryptfs would really like to have read-write access to all files in the
lower filesystem.  Right now, the persistent lower file may be opened
read-only if the attempt to open it read-write fails.  One way to keep
from having to do that is to have a privileged kthread that can open the
lower persistent file on behalf of the user opening the eCryptfs file;
this patch implements this functionality.

This patch will properly allow a less-privileged user to open the eCryptfs
file, followed by a more-privileged user opening the eCryptfs file, with
the first user only being able to read and the second user being able to
both read and write.  eCryptfs currently does this wrong; it will wind up
calling vfs_write() on a file that was opened read-only.  This is fixed in
this patch.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Dave Kleikamp <shaggy@austin.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ecryptfs/Makefile

@@ -4,4 +4,4 @@
 
 obj-$(CONFIG_ECRYPT_FS) += ecryptfs.o
 
-ecryptfs-objs := dentry.o file.o inode.o main.o super.o mmap.o read_write.o crypto.o keystore.o messaging.o netlink.o miscdev.o debug.o
+ecryptfs-objs := dentry.o file.o inode.o main.o super.o mmap.o read_write.o crypto.o keystore.o messaging.o netlink.o miscdev.o kthread.o debug.o

fs/ecryptfs/ecryptfs_kernel.h

@@ -559,6 +559,20 @@ extern struct kmem_cache *ecryptfs_key_record_cache;
 extern struct kmem_cache *ecryptfs_key_sig_cache;
 extern struct kmem_cache *ecryptfs_global_auth_tok_cache;
 extern struct kmem_cache *ecryptfs_key_tfm_cache;
+extern struct kmem_cache *ecryptfs_open_req_cache;
+
+struct ecryptfs_open_req {
+#define ECRYPTFS_REQ_PROCESSED 0x00000001
+#define ECRYPTFS_REQ_DROPPED   0x00000002
+#define ECRYPTFS_REQ_ZOMBIE    0x00000004
+	u32 flags;
+	struct file **lower_file;
+	struct dentry *lower_dentry;
+	struct vfsmount *lower_mnt;
+	wait_queue_head_t wait;
+	struct mutex mux;
+	struct list_head kthread_ctl_list;
+};
 
 int ecryptfs_interpose(struct dentry *hidden_dentry,
 		       struct dentry *this_dentry, struct super_block *sb,
@@ -690,5 +704,10 @@ void ecryptfs_msg_ctx_alloc_to_free(struct ecryptfs_msg_ctx *msg_ctx);
 int
 ecryptfs_spawn_daemon(struct ecryptfs_daemon **daemon, uid_t euid,
 		      struct user_namespace *user_ns, struct pid *pid);
+int ecryptfs_init_kthread(void);
+void ecryptfs_destroy_kthread(void);
+int ecryptfs_privileged_open(struct file **lower_file,
+			     struct dentry *lower_dentry,
+			     struct vfsmount *lower_mnt);
 
 #endif /* #ifndef ECRYPTFS_KERNEL_H */

fs/ecryptfs/file.c

@@ -192,6 +192,13 @@ static int ecryptfs_open(struct inode *inode, struct file *file)
 				      | ECRYPTFS_ENCRYPTED);
 	}
 	mutex_unlock(&crypt_stat->cs_mutex);
+	if ((ecryptfs_inode_to_private(inode)->lower_file->f_flags & O_RDONLY)
+	    && !(file->f_flags & O_RDONLY)) {
+		rc = -EPERM;
+		printk(KERN_WARNING "%s: Lower persistent file is RO; eCryptfs "
+		       "file must hence be opened RO\n", __func__);
+		goto out;
+	}
 	ecryptfs_set_file_lower(
 		file, ecryptfs_inode_to_private(inode)->lower_file);
 	if (S_ISDIR(ecryptfs_dentry->d_inode->i_mode)) {

fs/ecryptfs/kthread.c

@@ -0,0 +1,203 @@
+/**
+ * eCryptfs: Linux filesystem encryption layer
+ *
+ * Copyright (C) 2008 International Business Machines Corp.
+ *   Author(s): Michael A. Halcrow <mahalcro@us.ibm.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#include <linux/kthread.h>
+#include <linux/freezer.h>
+#include <linux/wait.h>
+#include <linux/mount.h>
+#include "ecryptfs_kernel.h"
+
+struct kmem_cache *ecryptfs_open_req_cache;
+
+static struct ecryptfs_kthread_ctl {
+#define ECRYPTFS_KTHREAD_ZOMBIE 0x00000001
+	u32 flags;
+	struct mutex mux;
+	struct list_head req_list;
+	wait_queue_head_t wait;
+} ecryptfs_kthread_ctl;
+
+static struct task_struct *ecryptfs_kthread;
+
+/**
+ * ecryptfs_threadfn
+ * @ignored: ignored
+ *
+ * The eCryptfs kernel thread that has the responsibility of getting
+ * the lower persistent file with RW permissions.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int ecryptfs_threadfn(void *ignored)
+{
+	set_freezable();
+	while (1)  {
+		struct ecryptfs_open_req *req;
+
+		wait_event_freezable(
+			ecryptfs_kthread_ctl.wait,
+			(!list_empty(&ecryptfs_kthread_ctl.req_list)
+			 || kthread_should_stop()));
+		mutex_lock(&ecryptfs_kthread_ctl.mux);
+		if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
+			mutex_unlock(&ecryptfs_kthread_ctl.mux);
+			goto out;
+		}
+		while (!list_empty(&ecryptfs_kthread_ctl.req_list)) {
+			req = list_first_entry(&ecryptfs_kthread_ctl.req_list,
+					       struct ecryptfs_open_req,
+					       kthread_ctl_list);
+			mutex_lock(&req->mux);
+			list_del(&req->kthread_ctl_list);
+			if (!(req->flags & ECRYPTFS_REQ_ZOMBIE)) {
+				dget(req->lower_dentry);
+				mntget(req->lower_mnt);
+				(*req->lower_file) = dentry_open(
+					req->lower_dentry, req->lower_mnt,
+					(O_RDWR | O_LARGEFILE));
+				req->flags |= ECRYPTFS_REQ_PROCESSED;
+			}
+			wake_up(&req->wait);
+			mutex_unlock(&req->mux);
+		}
+		mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	}
+out:
+	return 0;
+}
+
+int ecryptfs_init_kthread(void)
+{
+	int rc = 0;
+
+	mutex_init(&ecryptfs_kthread_ctl.mux);
+	init_waitqueue_head(&ecryptfs_kthread_ctl.wait);
+	INIT_LIST_HEAD(&ecryptfs_kthread_ctl.req_list);
+	ecryptfs_kthread = kthread_run(&ecryptfs_threadfn, NULL,
+				       "ecryptfs-kthread");
+	if (IS_ERR(ecryptfs_kthread)) {
+		rc = PTR_ERR(ecryptfs_kthread);
+		printk(KERN_ERR "%s: Failed to create kernel thread; rc = [%d]"
+		       "\n", __func__, rc);
+	}
+	return rc;
+}
+
+void ecryptfs_destroy_kthread(void)
+{
+	struct ecryptfs_open_req *req;
+
+	mutex_lock(&ecryptfs_kthread_ctl.mux);
+	ecryptfs_kthread_ctl.flags |= ECRYPTFS_KTHREAD_ZOMBIE;
+	list_for_each_entry(req, &ecryptfs_kthread_ctl.req_list,
+			    kthread_ctl_list) {
+		mutex_lock(&req->mux);
+		req->flags |= ECRYPTFS_REQ_ZOMBIE;
+		wake_up(&req->wait);
+		mutex_unlock(&req->mux);
+	}
+	mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	kthread_stop(ecryptfs_kthread);
+	wake_up(&ecryptfs_kthread_ctl.wait);
+}
+
+/**
+ * ecryptfs_privileged_open
+ * @lower_file: Result of dentry_open by root on lower dentry
+ * @lower_dentry: Lower dentry for file to open
+ * @lower_mnt: Lower vfsmount for file to open
+ *
+ * This function gets a r/w file opened againt the lower dentry.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+int ecryptfs_privileged_open(struct file **lower_file,
+			     struct dentry *lower_dentry,
+			     struct vfsmount *lower_mnt)
+{
+	struct ecryptfs_open_req *req;
+	int rc = 0;
+
+	/* Corresponding dput() and mntput() are done when the
+	 * persistent file is fput() when the eCryptfs inode is
+	 * destroyed. */
+	dget(lower_dentry);
+	mntget(lower_mnt);
+	(*lower_file) = dentry_open(lower_dentry, lower_mnt,
+				    (O_RDWR | O_LARGEFILE));
+	if (!IS_ERR(*lower_file))
+		goto out;
+	req = kmem_cache_alloc(ecryptfs_open_req_cache, GFP_KERNEL);
+	if (!req) {
+		rc = -ENOMEM;
+		goto out;
+	}
+	mutex_init(&req->mux);
+	req->lower_file = lower_file;
+	req->lower_dentry = lower_dentry;
+	req->lower_mnt = lower_mnt;
+	init_waitqueue_head(&req->wait);
+	req->flags = 0;
+	mutex_lock(&ecryptfs_kthread_ctl.mux);
+	if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
+		rc = -EIO;
+		mutex_unlock(&ecryptfs_kthread_ctl.mux);
+		printk(KERN_ERR "%s: We are in the middle of shutting down; "
+		       "aborting privileged request to open lower file\n",
+			__func__);
+		goto out_free;
+	}
+	list_add_tail(&req->kthread_ctl_list, &ecryptfs_kthread_ctl.req_list);
+	mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	wake_up(&ecryptfs_kthread_ctl.wait);
+	wait_event(req->wait, (req->flags != 0));
+	mutex_lock(&req->mux);
+	BUG_ON(req->flags == 0);
+	if (req->flags & ECRYPTFS_REQ_DROPPED
+	    || req->flags & ECRYPTFS_REQ_ZOMBIE) {
+		rc = -EIO;
+		printk(KERN_WARNING "%s: Privileged open request dropped\n",
+		       __func__);
+		goto out_unlock;
+	}
+	if (IS_ERR(*req->lower_file)) {
+		rc = PTR_ERR(*req->lower_file);
+		dget(lower_dentry);
+		mntget(lower_mnt);
+		(*lower_file) = dentry_open(lower_dentry, lower_mnt,
+					    (O_RDONLY | O_LARGEFILE));
+		if (IS_ERR(*lower_file)) {
+			rc = PTR_ERR(*req->lower_file);
+			(*lower_file) = NULL;
+			printk(KERN_WARNING "%s: Error attempting privileged "
+			       "open of lower file with either RW or RO "
+			       "perms; rc = [%d]. Giving up.\n",
+			       __func__, rc);
+		}
+	}
+out_unlock:
+	mutex_unlock(&req->mux);
+out_free:
+	kmem_cache_free(ecryptfs_open_req_cache, req);
+out:
+	return rc;
+}

fs/ecryptfs/main.c

@@ -130,26 +130,12 @@ static int ecryptfs_init_persistent_file(struct dentry *ecryptfs_dentry)
 			ecryptfs_dentry_to_lower_mnt(ecryptfs_dentry);
 
 		lower_dentry = ecryptfs_dentry_to_lower(ecryptfs_dentry);
-		/* Corresponding dput() and mntput() are done when the
+		rc = ecryptfs_privileged_open(&inode_info->lower_file,
-		 * persistent file is fput() when the eCryptfs inode
+						     lower_dentry, lower_mnt);
-		 * is destroyed. */
+		if (rc || IS_ERR(inode_info->lower_file)) {
-		dget(lower_dentry);
-		mntget(lower_mnt);
-		inode_info->lower_file = dentry_open(lower_dentry,
-						     lower_mnt,
-						     (O_RDWR | O_LARGEFILE));
-		if (IS_ERR(inode_info->lower_file)) {
-			dget(lower_dentry);
-			mntget(lower_mnt);
-			inode_info->lower_file = dentry_open(lower_dentry,
-							     lower_mnt,
-							     (O_RDONLY
-							      | O_LARGEFILE));
-		}
-		if (IS_ERR(inode_info->lower_file)) {
 			printk(KERN_ERR "Error opening lower persistent file "
-			       "for lower_dentry [0x%p] and lower_mnt [0x%p]\n",
+			       "for lower_dentry [0x%p] and lower_mnt [0x%p]; "
-			       lower_dentry, lower_mnt);
+			       "rc = [%d]\n", lower_dentry, lower_mnt, rc);
 			rc = PTR_ERR(inode_info->lower_file);
 			inode_info->lower_file = NULL;
 		}
@@ -679,6 +665,11 @@ static struct ecryptfs_cache_info {
 		.name = "ecryptfs_key_tfm_cache",
 		.size = sizeof(struct ecryptfs_key_tfm),
 	},
+	{
+		.cache = &ecryptfs_open_req_cache,
+		.name = "ecryptfs_open_req_cache",
+		.size = sizeof(struct ecryptfs_open_req),
+	},
 };
 
 static void ecryptfs_free_kmem_caches(void)
@@ -795,11 +786,17 @@ static int __init ecryptfs_init(void)
 		printk(KERN_ERR "sysfs registration failed\n");
 		goto out_unregister_filesystem;
 	}
+	rc = ecryptfs_init_kthread();
+	if (rc) {
+		printk(KERN_ERR "%s: kthread initialization failed; "
+		       "rc = [%d]\n", __func__, rc);
+		goto out_do_sysfs_unregistration;
+	}
 	rc = ecryptfs_init_messaging(ecryptfs_transport);
 	if (rc) {
-		ecryptfs_printk(KERN_ERR, "Failure occured while attempting to "
+		printk(KERN_ERR "Failure occured while attempting to "
 				"initialize the eCryptfs netlink socket\n");
-		goto out_do_sysfs_unregistration;
+		goto out_destroy_kthread;
 	}
 	rc = ecryptfs_init_crypto();
 	if (rc) {
@@ -814,6 +811,8 @@ static int __init ecryptfs_init(void)
 	goto out;
 out_release_messaging:
 	ecryptfs_release_messaging(ecryptfs_transport);
+out_destroy_kthread:
+	ecryptfs_destroy_kthread();
 out_do_sysfs_unregistration:
 	do_sysfs_unregistration();
 out_unregister_filesystem:
@@ -833,6 +832,7 @@ static void __exit ecryptfs_exit(void)
 		printk(KERN_ERR "Failure whilst attempting to destroy crypto; "
 		       "rc = [%d]\n", rc);
 	ecryptfs_release_messaging(ecryptfs_transport);
+	ecryptfs_destroy_kthread();
 	do_sysfs_unregistration();
 	unregister_filesystem(&ecryptfs_fs_type);
 	ecryptfs_free_kmem_caches();